Patagonian Aridification at the Onset of the Mid-Miocene Climatic Optimum

Fossil-rich sediments of the Santa Cruz Formation, Patagonia, Argentina, span the initiation of the Miocene Climatic Optimum (MCO), the most recent period of warm and wet conditions in the Cenozoic. These conditions drove the expansion of tropical and subtropical ecosystems to much higher latitudes, with the fossiliferous Santa Cruz Formation recording one of the southernmost examples. We collected new carbon and oxygen isotope compositions of herbivore tooth enamel from fossils similar to 17.4 to 16.4 Ma in age to investigate ecological and climatic changes across the initiation of the MCO. Enamel delta C-13 values are consistent with a C-3-dominated ecosystem with moderate precipitation and a mix of wooded and more open areas. Serially sampled teeth reveal little zoning in delta C-13 and delta O-18 values, suggesting little seasonal variation in water and plant isotope compositions or seasonal changes in diet. Carbon isotope-based estimates of mean annual precipitation (MAP) are consistent with aridification, with MAP decreasing from similar to 1,000 +/- 235 mm/yr at 17.4 Ma to similar to 525 +/- 105 mm/yr at the start of the climatic optimum (similar to 16.9 Ma). This decrease corresponds to increasing global temperatures, as indicated by marine proxy records, and was followed by a rebound to similar to 840 +/- 270 mm/yr by similar to 16.4 Ma. In comparison to a modern mean annual temperature (MAT) in the region of similar to 8 degrees C, oxygen isotopes indicate high MAT (at least 20 degrees C) at the onset of the MCO at 16.9 Ma and a significant increase in MAT to similar to 25 degrees C by 16.4 Ma.